Milling trigonal selenium particles to improve xerographic performance

ABSTRACT

A method of improving the photoconductive properties of trigonal selenium which comprises first providing a source of unmilled particulate trigonal selenium having an average particle size usually greater than about 1 micron. The particulate trigonal selenium is then milled to an average particle size of less than about 1 micron, whereby the selenium particles exhibit improved xerographic properties.

United States Patent Karam et al. 1 Oct. 7, 1975 [54] MILLING TRIGONAL SELENIUM 3,787,208 1/1974 Jones 1 1 1 v 117/34 3,837 906 9/l974 101165 1 1 1 4 v 1 A 1 s 96/l.5 PERFORMANCE [75] lnvemors: R0na|d Karam Webster NY; Primary ExaminerGranville Y. Custer, Jr.

David Swanhout, Arlington Arlorney, Agent, or Firm.lames J Ralabate; James P. Mass OSullivan; Jerome L. Jeffers [73] Assignee: Xerox Corporation, Stamford,

57 ABSTRACT [22] Filed: June 21,1974 I A method of improving the photoconducuve proper i 1 PP NOJ 48L586 ties of trigonal selenium which comprises first providing a source of unmilled particulate trigonal selenium [52] U5. CL I I I I I H 423/510; 241/27. 965 having an average particle size usually greater ihan [5|] In. H C01B 19/00; B02C 19/12 about I micron. The particulate trigonal selenium is [58] Fidd of Search u 241/27 96/15. then milled to an average particle size of less than 423/510. 201 about 1 micron, whereby the selenium particles exhibit improved xerographic properties.

[56] References Cited UNITED STATES PATENTS 7 Clalms, 6 Drawing Flgures 2 66Z.832 12/1953 Middleton el al. 423/5l0 X US. Patent 0a. 7,1975 Sheet 1 of2 3,911,091

FIG 3 FIG. 5

U.S. Patent Oct. 7,1975 Sheet 2 of2 3,911,091

VOLTS 600 200 ET1T lllllllli,

SECONDS FIG. 6

MILLING TRIGONAL SELENIUM PARTICLES TO IMPROVE XEROGRAPHIC PERFORMANCE BACKGROUND OF THE INVENTION This invention relates in general to xerography and more specifically to a method of improving the photosensitivity of finely divided trigonal selenium particles suitable for use in a photoconductive binder layer.

In the art of xerography, a xerographic plate containing a photoconductive insulating layer is first given a uniform electrostatic charge in order to sensitize the surface of the photoconductive layer. The plate is then exposed to an image of activating electromagnetic radiation, such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulator while leaving behind the latent electrostatic image in the nonilluminated areas. The latent electrostatic image may be developed and made visible by depositing finely divided electroscopic marking particles on the surface of the photoconductive layer. This concept was originally described by Carlson in U.S. Pat. No. 2,297,69l and is further amplified and described by many related patents in the field.

Conventional xerographic plates or drums usually comprise a photoconductive insulating layer overlaying a conductive support. A photoconductive material which has had wide use as a reusable photoconductor in commercial xerography comprises vitreous or amorphous selenium. Vitreous selenium in essence comprises super cooled selenium liquid and may readily be formed by vacuum evaporation by cooling the liquid or vapor so suddenly that crystals of selenium do not have time to form. Although vitreous selenium has had wide acceptance for commercial use in xerography, its spectral response is limited largely to the blue-green portion of the electromagnetic spectrum which is below about 5200 Angstrom Units. In addition, the preparation of vitreous selenium by vacuum deposition requires a significant capital expenditure for vacuum coating apparatus and closely controlled process parameters are required in order to obtain a photoconductive layer having the desired electrical characteristics. In general, one requirement of a photoconductor, such as vitreous selenium, is that its resistivity should drop at least several orders of magnitude in the presence of activating radiation or light in comparison to its resistivity in the dark. Also, the photoconductive layers should be able to support a significant electrical potential in the absence of radiation Selenium also exists in a crystalline form known as trigonal or hexagonal selenium which is well known to the semiconductor art for use in the manufacture of selenium rectifiers. In the crystalline trigonal form, the structure of the selenium consists of helical chains of selenium atoms which are parallel to each other along the crystallographic c-axis. Trigonal selenium is not normally used in xerography as a homogeneous photoconductive layer because ofits relatively high electrical conductivity in the dark, although in some instances trigonal selenium can be used in binder structures where trigonal selenium particles are dispersed in a matrix of another material such as an electrically insulating resin, an electrically active organic material, or a photoconductor such as vitreous selenium.

U.S. Pat. Nos. 2,739,079 and 3,692,521 both describe photosensitive members utilizing small amounts of crystalline hexagonal (trigonal) selenium contained in predominantly vitreous selenium matrices. In addition, copending U.S. patent application Ser. No. 669,915, filed Sept. 22, 1967, describes a special form of red-hexagonal selenium suitable for use in binder structures in which finely divided red-hexagonal sele nium particles are contained in a resin binder matrix.

Although trigonal selenium exhibits a Wider spectral response and is more thermally stable than vitreous selenium, as stated above, trigonal selenium is not normally used in xerography because of its relatively high electrical conductivity in the dark. However, imaging members which are able to use hexagonal selenium would have advantages over those using vitreous selenium with regard to improved spectral response. Further, the use of trigonal selenium in xerographic members, especially in the binder form, would provide greater ease in the manufacture of the photoconductive device in that the expensive vacuum coating apparatus required for forming vitreous selenium would not be necessary in forming a binder layer containing trigonal selenium particles. Binder layers are also inherently more flexible than evaporated layers. In addition, solvent coated binder layers can adhere more tenaciously to substrates than vacuum evaporated layers.

OBJECTS OF THE INVENTION It is therefore an object of this invention to provide a method of improving the photosensitivity and other xerographic properties of finely divided trigonal selenium particles.

It is yet another object of this invention to provide a photosensitive particulate trigonal selenium material suitable for use in photosensitive imaging members,

SUMMARY OF THE INVENTION The present invention is directed to a method of improving the xerographic properties of particulate trigonal selenium. This invention is based upon the discovery that when trigonal selenium particles are milled under certain conditions, significant improvements in both the photoinduced discharge curve and charge acceptance are observed as compared to trigonal selenium particles which have not been milled. This effect is observed when the trigonal selenium is employed in a photoconductive binder layer in a composite imaging device which will be later described in greater detail.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1-5 represent five different embodiments of imaging structures suitable for using particulate trigonal selenium of the present invention.

FIG. 6 represents a plot with the photo-discharge characteristics of milled trigonal selenium of the present invention compared to unmilled trigonal selenium.

DETAILED DESCRIPTION OF THE DRAWINGS The particulate trigonal selenium of the present invention can be utilized in a variety of imaging structures. illustrated more clearly by FIGS. 1-5 of the drawings. FIG. 1 comprises an imaging member I0 having a conductive substrate II overcoated with a binder photogenerating layer 12 comprising trigonal selenium particles I3 dispersed in a matrix I4 which usually comprises an electrically active polymer such as polyvinyl carbazole (PVK), polyvinyl pyrene (PVP), or 2,4,7-trinitro 9-fluorenone (TNF); or a combination of PVK or PVP, with TNF or similar compounds. A transport layer overlays the photogenerating binder layer and comprises an electrically active material such as polyvinyl carbazole. polyvinyl pyrene, PVK or PVP with TNF. When using TNF or a transport layer it is preferred that the material be blended with a polymer in order to enhance the mechanical properties of the transport layer. Under the influence of an electrical field. such polymers are capable of transporting a photoinjected charge from the photogenerating layer and hence are referred to as active polymers. A satisfactory thickness for the binder photoinjecting layer is from about 0.5 to 6 microns. The thickness of the active transport layer is from about 5 to 100 microns, but thicknesses outside this range can also be used. Preferably, a thickness of about 5 to 25 microns provides particularly satisfactory results. These structures are more fully defined and described in detail in copending application Ser. No. 371,647, filed June 20, 1973, which is incorporated herein by reference.

The imaging member of FIG. 2 is similar to the member of FIG. I except that substrate 21 is overcoated with photoinjecting layer 22 comprising the same trigonal selenium particles 23 in substantial particleto-particle contact contained in an electrically insulating resin matrix 24 such as a silicone resin or polyester. Alternatively, the insulating resin may be replaced with an electrically active material of the type described for FIG. 1 above. The transport overlayer 25 is electrically active and identical or equivalent to the materials described for the transport layer 15 of FIG. I. In operation, the imaging members of FIGS. 1 and 2 are normally uniformly electrostatically charged and then imaged by exposure to a pattern of light to which the top transport layer is substantially nonabsorbing or transparent. Charge carriers are generated by the photogenerating layer, and injected into and transported through the transport layer to selectively discharge a surface charge on top of the transport layer.

The imaging members and of FIGS. 3 and 4, respectively. are directed to alternative embodiments of FIGS. 1 and 2, respectively, in which the photogenerating layer is contained on top of the transport layer. More specifically, in FIG. 3, conductive substrate 31 is overcoated with a layer of active organic material 32 which contains a top binder layer 33 comprising trigonal selenium particles 34 contained in an electrically active matrix 35. Similarly, FIG. 4 is an alternative embodiment of FIG. 3 in which conductive substrate 41 overlayed with a transport layer 42, contains a binder layer 43 in which the trigonal selenium particles 44 are in substantial particle-to-particle contact and contained in a matrix of electrically insulating material 45 or an active material such PVK, PVP, PVK or PVP and TNF. In operation the imaging members of FIGS. 3 and 4 are uniformly electrostatically charged to a given polarity and then imaged with light to which the top photogenerating layer is absorbing. The charge car riers generated by the top layer are injected into and transported through the middle transport layer, while an opposite charge dissipates the electrostatic charge at the surface of the top layer. In this case, the transport layer need not be transparent to light since most otthe light is absorbed in the generator layer.

In another embodiment of the present invention, illustrated in FIG. 5, imaging member comprises a single binder layer 52 formed on conductive substrate 5] Binder layer 52 comprises a relatively small amount of photoconductive trigonal selenium 53 contained in an electrically active matrix 54 which may comprise a material such as polyvinyl carbazole or polyvinyl pyrene. These materials may be used in combination with other materials such as TNF in order to improve the cycling characteristics of the imaging member.

DESCRIPTION OF THE PREFERRED EMBODIMENT The following examples further specifically define the present invention with respect to a method of manufacturing and testing milled trigonal selenium particles. Unless otherwise mentioned, parts and percentages in the examples are expressed by weight.

EXAMPLE I Twelve strips of anodized aluminum are each overcoated with a 2 micron photogenerator binder layer containing 50 percent powdered trigonal selenium by volume.

Six of these layers are made from the selenium which has been converted to the trigonal form before incorporation into the generator layer. The trigonal selenium used in this study is prepared by heating amorphous selenium under vacuum at I25 for l hour and rapidly quenching it. This selenium is then ground with a mortar and pestle, and passed through a I00 mesh sieve. The selenium particles have a size distribution of about I to microns and an average particle size of about 14 microns. About 0.378 gram of this selenium is placed in a 2 oz. glassjar with 40 grams of /2 inch diameter steel shot in the presence of ().l gram of PVK and 5 ml. of chloroform. This mixture is then milled for 4 hours on a paint shaker. In this way, the trigonal selenium is severely milled or work damaged during the fabrication procedure. The six plates prepared by this procedure were then heated under a vacuum of approximately l0" Torr at 125 for 18 hours and slowly quenched to room temperature.

The six remaining layers are made from powdered amorphous selenium dispersed in PVK. These layers are also heated under vacuum at 125 for IS hours and slowly cooled. In this way. the amorphous selenium is converted to the trigonal form in situ. The selenium formed by this procedure is, therefore, not milled or work damaged in any way after conversion to the trigonal form.

All l2 layers are then overcoated with approximately a l4 micron layer of PVK, dried under vacuum at for 16 hours and slowly cooled. This experiment produced two sets of plates:

Set I contains structures with trigonal selenium binder layers which had been converted to the trigonal form in situ and therefore not milled.

Set II contains structures with selenium which had been converted to the trigonal form prior to its incorporation into the composite imaging structure. This trigonal selenium had been milled during the fabrication procedure Both sets of plates were examined xerographically with their photoinduced discharge curves being plotted in FIG. 6. It can be readily seen from the data in FIG. 6, that a significantly improved photodischarge curve is exhibited for the milled trigonal selenium imaging members of Set II, as opposed to the unmilled trigonal selenium imaging members of Set I.

The samples of Sets l and 11 were also examined using X-ray diffractometry. The trigonal selenium prepared in situ showed no preferential orientation, whereas the milled trigonal selenium exhibited some preferred orientation, with the crystallographic c-axis showing some 5 Other modifications and ramifications of the present preference to align in a direction parallel to the subinvention would appear to those skilled in the art upon Stratereading the disclosure. These are also intended to be EXAMPLE H within the scope of this invention.

An additional 4 imaging members are made by the What is Claimed is: method of Example 1. The average particle size after 1. A method of improving the photoconductive propmilling; generator layer thickness and charge acceperties of trigonal selenium which comprises providing tance (measured as field) at two different surface asource of unmilled particulate trigonal selenium par charge levels are tabulated in the Data Table below: ticles, said method comprising milling said particulate l5 DATA TABLE Approximate Field (volts/ Field (volts/ Aurrage Generator microns) at microns) at Particle Layer (J, *=(l.77 X 10 0: *=1 28 X 10 Sample Size [In Thickness coul coul Nu. Microns) (In Microns) cm cm" l 2 30.5 41.2; 3 -u75 19.6 27.5 3 -02 2.5 4.5 30.2 38.7 4 4:75 2 4 I41 16.6

' Q 7 surface charge it can be seen from the results in the Data Table that samples which were milled to a smaller size, could be charged to higher fields.

Although the degree of milling is critical, the milling conditions cannot readily be expressed in process parameters. Rather, it appears that the degree of reduction in particle size is a factor of considerable importance. Broadly, the invention includes taking unmilled trigonal selenium particles greater than about 1 micron in size and milling said particles until they are less than about 1 micron in size, and preferably about 0.2 microns or less in size. For example. starting with trigonal selenium particles having a size distribution of about 1 to 70 microns and an average particle size of about 14 microns, milling which results in reducing that particle size to between about 0.05 and 0.5 microns has been found satisfactory in order to yield the improved photosensitivity observed for the present invention.

In another embodiment of the present invention, under certain circumstances, the xerographic properties of finely divided trigonal selenium can also be improved by milling Without a corresponding decrease in particle size. For example, when milling submicron size previously unmilled trigonal selenium particles under substantially the same conditions as in Example I, no reduction in particle size was observed, but the milled trigonal selenium particles exhibited improved charge acceptance.

trigonal selenium to reduce the original particle size whereby said particles exhibit improved photosensitivity.

2. The product formed by the process of claim 1.

3. A method of improving the photoconductive properties of trigonal selenium which comprises providing a source of unmilled particulate trigonal selenium having an average particle size greater than about 1 micron, said method comprising milling said particulate trigonal selenium to an average particle size of less than about 1.0 micron, whereby said particles exhibit improved photosensitivity.

4. A method ofimproving the photoconductive properties of trigonal selenium which comprises providing a source of unmilled particulate trigonal selenium having an average particle size greater than about 1 micron, said method comprising milling said particulate trigonal selenium to an average particle size of about 0.2 microns or less whereby said particles exhibit improved photosensitivity.

5. The product formed by the process of claim 4.

6. A method of improving the photoconductive properties of trigonal selenium which comprises providing a source of unmilled particulate trigonal selenium, said method comprising milling said particulate trigonal selenium without reducing the particle size of the particles whereby said particles exhibit improved photosensitivity.

7. The product formed by the process of claim l. 

1. A METHOD OF IMPROVING THE PHOTOCONDUCTIVE PROPERTIES OF TRIGONAL SELENIUM WHICH COMPRISES PROVIDING A SOURCE OF UNMILLED PARTICULATE TRIGONAL SELENIUM PARTICLES, SAID METHOD COMPRISING MILLIMG SAID PARTICULATE TRIGONAL SELENIUM TO REDUCE THE ORIGINAL PARTICLE SIZE WHEREBY SAID PARTICLES EXHIBIT IMPROVED PHOTOSENSITIVITY.
 2. The product formed by the process of claim
 1. 3. A method of improving the photoconductive properties of trigonal selenium which comprises providing a source of unmilled particulate trigonal selenium having an average particle size greater than about 1 micron, said method comprising milling said particulate trigonal selenium to an average particle size of less than about 1.0 micron, whereby said particles exhibit improved photosensitivity.
 4. A method of improving the photoconductive properties of trigonal selenium which comprises providing a source of unmilled particulate trigonal selenium having an average particle size greater than about 1 micron, said method comprising milling said particulate trigonal selenium to an average particle size of about 0.2 microns or less whereby said particles exhibit improved photosensitivity.
 5. The product formed by the process of claim
 4. 6. A method of improving the photoconductive properties of trigonal selenium which comprises providing a source of unmilled particulate trigonal selenium, said method comprising milling said particulate trigonal selenium without reducing the particle size of the particles whereby said particles exhibit improved photosensitivity.
 7. The product formed by the process of claim
 1. 